Carry a re-engraved slice's notation in the bundle

A replaced slice shipped as pixels only, so the LilyPond behind it died
with the project file — and a project is spent once exported. Correcting
one wrong note meant retyping the system.

Slices gain an optional `engraving` object: the language, key, time and
one entry per voice holding the notes and lyrics verbatim. Structured per
staff rather than one blob of source, because that is what both a later
edit and a MIDI render want; song-level key and time are resolved per
slice so reading one needs no context from its neighbours.

Additive and ignorable, so the format stays at version 1.

tests/test_markers.py

@@ -22,6 +22,8 @@ from noteman_slicer.project import (  # noqa: E402
     Cut,
     Marker,
     Project,
+    Replacement,
+    Voice,
     default_path,
 )
 
@@ -91,6 +93,29 @@ def main() -> int:
     assert slices[1]["markers"][0] == {"type": "coda"}
     assert slices[0]["markers"][1] == {"type": "to_coda", "destination": 1}
 
+    # A re-engraved slice carries its notation into the bundle; a scanned one
+    # carries none. This is what makes a later edit or a MIDI render possible
+    # from the bundle alone.
+    project.key, project.time = "aes", "3/4"
+    page.replacements[second] = Replacement(
+        voices=[Voice("treble", "c4 d e f", "la la la la"), Voice("bass", " c4 d e f ", "  ")]
+    )
+    engraved = song_json(project, names)["slices"]
+    assert "engraving" not in engraved[0], "a scanned slice has no notation"
+    ly = engraved[1]["engraving"]
+    assert ly["lang"] == "lilypond"
+    # Song defaults are resolved per slice: reading one slice needs no context.
+    assert (ly["key"], ly["time"], ly["print_time"]) == ("aes", "3/4", False)
+    assert ly["voices"][0] == {"clef": "treble", "notes": "c4 d e f", "lyrics": "la la la la"}
+    assert "lyrics" not in ly["voices"][1], "an empty field is absent, not empty"
+    assert ly["voices"][1]["notes"] == "c4 d e f"
+
+    override = Replacement(voices=page.replacements[second].voices, key="d", print_time=True)
+    page.replacements[second] = override
+    ly = song_json(project, names)["slices"][1]["engraving"]
+    assert (ly["key"], ly["time"], ly["print_time"]) == ("d", "3/4", True)
+    page.replacements[second] = None
+
     # A jump whose target got discarded is dropped, not exported dangling.
     project.pages[0].discards[second] = True
     dropped = song_json(project, ["001.webp"])